You could logic probe them and see which gates are stuck. There is even a spare gate on the digit strobe section if it is just one bad one.

How does your clock signal look on pin 9 of z22?

You can see if it is moving with just a meter. Set it to DCv and probe that pin on Z22, make sure it isn't stuck high or low. It should look like it is not settling on one voltage.

Well that at least rules out the incoming clock signal. The only way to really debug this now without shotgunning a lot more stuff further upstream at this point would be with a scope or logic tester to see what the actual data being sent to the strobe decoder and flip-flops it doing. You could try probing it the same way with a meter, but no guarantee you'll see anything because it is moving too quickly. But you really want to eliminate guess work if you can.

Something like this would be cheaper than a scope to rule stuff out. Goes up to 20mhz
https://www.amazon.com/Elenco-Electronics-LP-560-Logic-Probe/dp/B000Z9HAP4/ref=sr_1_2

Since RIOT chips are so difficult to source and pricey, it might not be a bad idea to have another tool available to mitigate growing costs of parts.

I'd also go back to the simpler stuff like the physical wiring & connection to that display. Since the other stuff is working and that is the only remote display (minus the apron), there is a good chance it is something really simple like an internal wire break that you just can't see. Have you messed with continuity testing over the connector cable at all?

If you're gonna shotgun anything next, that is the part to replace.

Hopefully that takes care of this. I'm emotionally invested in this one now, ha! I love vintage computing because you can actually make sense of what is going on in real time.

Time for a logic probe purchase!

Excellent. That'll solve the connector question for sure.

This is one of those situations where I wish I weren't right for your sake. There should be info in the schematics regarding that connector. If I had a copy, I'd look through it for you. That could also just be a service connector. I don't know a ton about Gottlieb stuff admittedly. I got to work on a Black Hole once a few years back for like an hour or so. That is the extent of my experience with it.

And this is why this forum is awesome. That was quick.

You sure you don't have the special version of this game bundy?

blackhole-(1) (resized).jpg